Ultrasonic techniques are commonly used for non-destructive testing or evaluation (NDT or NDE) of materials, e.g., by measurement of properties of the material such as ultrasonic velocity and attenuation. One area in which such ultrasonic techniques are applied is the evaluation of underground/undersea geological formations for determining the presence or absence of hydrocarbons (e.g., petroleum and natural gas) and the evaluation of the quality of casing and cement used in boreholes drilled in such underground/undersea geological formations for discovering and extracting such hydrocarbons.
In such ultrasonic techniques, transducers are commonly used to transmit acoustic signals into the specimen of interest (material under test) and to receive responses to the transmitted signals, e.g., reflections or echoes thereof, the characteristics of which are analyzed to yield information about the properties of material under test. Under certain adverse conditions, e.g., under high temperature or pressure, in a corrosive environment, or when the specimen is very small, it is not feasible to use a transducer in direct contact with the specimen. Such adverse conditions may obtain in the underground/undersea exploration for hydrocarbons described above.
One conventional way of mitigating or overcoming such adverse conditions is by using a buffer rod between the transducer and the specimen. The buffer rod eliminates direct contact between the transducer and the specimen and thus protects the transducer from the adverse conditions present in the specimen environment. When acoustic properties of the buffer rod are known, these known properties in combination with the acoustic response can help in determining acoustic properties of the specimen. However, buffer rods suffer from the problem of spurious (trailing) echoes that interfere with the signal of interest (the response signal described above) received by the transducer. As these spurious echoes are caused at least in significant part by mode conversion at the buffer rod boundaries, they have been mitigated or overcome by making the buffer rod large, tapered, or grooved. However, such buffer rods are too large for accommodation in a downhole tool (e.g., a wireline or logging while drilling (LWD) tool) such as is used in boreholes for exploring for hydrocarbons underground/undersea.
Accordingly, there is a need for improving the accuracy and sensitivity of ultrasonic techniques for evaluation of materials, such as with use of a buffer rod, where the techniques can be performed and the equipment required therefor can be accommodated in a downhole tool.